Table 2 Guidelines for assessing risk of bias – based on study participation and outcome measurement domains of the QUIPS tool [39]

 Does the study sample represent the population of interest on key characteristics sufficient to limit potential bias to the results?

Target population: The source population or population of interest is adequately described for key characteristics.

Sampling: The sampling frame and recruitment are adequately described, possibly including methods to identify the sample (number and type used, e.g. referral patterns in health care), period of recruitment, and place of recruitment (setting and geographic location).

The sampling frame and procedures used to sample subjects should not lead to selection of participants that are systematiclly different from eligible non-participants.

Inclusion criteria: Inclusion and exclusion criteria are adequately described (e.g. including explicit diagnostic criteria or “zero time” description). Inclusion/exclusion criteria should not select participants that are systematically different from eligible non-participants.

Baseline study population: The baseline study sample (i.e. individuals entering the study) is adequately described for key characteristics.

Adequate study participation: There is adequate participation in the study by eligible individuals. Studies should report factors associated with non-response, quantify and interpret these associations to determine if it is a selective sample. For example, a low participation raises suspicion that there may be a barrier to participating that may influence outcomes.

 Is the outcome of interest adequately measured in study participants sufficeint to limit potential bias.

Definition of outcome: A clear definition of the outcome of interest is provided, including duration of follow-up and level and extent of the outcome construct.

Valid and reliable measure of outcome: The outcome measure and method used are adequately valid and reliable to limit misclassification bias (e.g., may include relevant outside sources of information on measurement properties, also characteristics, such as blind measurement and confirmation of outcome with valid and reliable test). Whenever possible, validated instruments should be used.

Method and setting of outcome measurement: The method and setting of measurement are the same for all study participants. The measurement approach, timing, and setting of assessment should be standardised across subjects, or conducted in a way that limits systematically different measurement. If there are differences, this should be reported and the implications should be considered.

Estimation of population parameters: Estimates of population parameters should be calculated using data observed in the whole sample, not extrapolated from rates observed in a sub-sample.